1 . Field of the Invention
The present invention relates to the field of gas turbines and in particular that of turbine engines and turboprops, for aircraft such as helicopters, airplanes, and other probable applications for these types of engine.
More particularly, the present invention relates to a turbine engine, in particular for a helicopter, the engine comprising a gas generator and a free turbine driven in rotation by a gas stream generated by the gas generator, the turbine engine further including a reversible electric machine for coupling to the gas generator, said reversible electric machine being suitable for setting the gas generator into rotation during a stage of starting the turbine engine.
2. Description of the Related Art
Conventionally, the gas generator comprises at least a compressor and a turbine that are coupled together in rotation. The principle of operation is as follows: fresh air entering into the turbine engine is compressed because of the rotation of the compressor, prior to being delivered to a combustion chamber where it is mixed with fuel. The gas burnt by the combustion is then exhausted at high speed.
This gas then expands initially in the turbine of the gas generator so that the turbine extracts therefrom the energy needed for driving the compressor.
The turbine of the gas generator does not absorb all of the kinetic energy of the burnt gas, and the remaining kinetic energy corresponds to the gas stream as generated by the gas generator.
This stream thus supplies kinetic energy to the free turbine such that the gas expands a second time in the free turbine, which serves to transform its kinetic energy into mechanical energy in order to drive a driven member, such as a helicopter rotor.
While starting the turbine engine, it is necessary to drive the gas generator in rotation, in other words to drive rotation of the compressor coupled to the turbine. As mentioned above, this is specifically one of the roles of the reversible electric machine, which is itself known from elsewhere, and which is usually an electric motor also suitable for operating in reversible manner as an electricity generator.
By using the reversible electric machine operating as a motor to drive rotation of the compressor, it is possible to cause air to flow through the compressor and thus to bring compressed air into the combustion chamber in order to initiate combustion.
The combustion then produces the gas stream that enables the turbine to be driven in rotation, whereupon rotation of the compressor is driven directly by the turbine, which means that the gas generator is operating in self-contained manner, in other words that the turbine engine has indeed started.
It is known that the aircraft in which such turbine engines are fitted include electrical equipment that needs to be powered with electricity.
For example, in a helicopter, it is necessary to provide electrical power for the electrical equipment with which it is fitted, such as for example electric controls, heating, air conditioning, a mechanical winch.
Until now, while in flight, the reversible electric machine has been used to deliver electricity for the electrical equipment. For this purpose, and as described in document EP 1 712 761, the electric machine operates as an electricity generator and is driven in rotation by the gas generator, with the rotary kinetic energy taken from the gas generator being transformed into electrical energy by said machine.
However, in a helicopter, taking kinetic energy from the gas generator involves drawbacks.
In flight, varying the amount of mechanical power taken off from the gas generator by the electric machine gives rise to a shift of the operating line of the engine in the compressor field.
This shift corresponds to a pumping margin that needs to be ensured, thus having the consequences:                of penalizing optimization of the engine operating line, by preventing the compressor being used at an optimum compression ratio; and        of thus degrading stabilized performance, with an impact on specific fuel consumption.        